Every state in America has regulations concerning the installation of "EXIT" signs in public buildings. In large part, these regulations are the result of work done by the "Committee On Safety To Life" of the National Fire Protection Association (NFPA), which was first appointed in 1913. For the first few years of its existence, the Committee devoted its attention to a study of notable fires involving loss of life, and to analyzing the causes of that loss of life. This work led to the preparation of standards for the construction of stairways, fire escapes, and, over the years, to the suggested embodiment of the exit signs themselves.
Signs designed for alerting the public to safe exit from a building in case of emergency can range from a simple flat, non-illuminated plastic decal to the elaborate lighted designs generally seen in larger buildings. In such applications, the law requires an illuminated sign with a battery back-up system to guard against failure in the event of a power outage. Where there are a large number of such signs, power usage becomes a real concern.
The first powered EXIT signs utilized incandescent or florescent bulbs for their illumination source. These signs were simple and straightforward in their design, comprising single or dual incandescent or fluorescent white light bulbs in a box, with one side an opaque panel with the word "EXIT" cut out. A colored plastic sheet was placed between the light source and this panel. The colored plastic sheet served the dual purpose of giving color to the light output and affording a degree of diffusion to the light in an attempt to avoid hot spots in the signs appearance.
This attempt at even illumination is a stated objective of the NFPA code, paragraph 5-10.3.4. This section of the code states that "Every sign required by 5-10.1.4 shall provide evenly illuminated letters having a minimum luminance of 0.06 footlamberts."
Another common approach was to paint the word "EXIT" in one color on a contrasting translucent panel, placed in front of a light source. Local codes may specify certain colors, but by convention, the letters are usually red on a green background. As the entire light source transmission is directed to the face of the sign, it is accepted that this creates a more noticeable display. It is also understood that with the display being in two contrasting colors, greater recognizability, particularly in smoky conditions, is realized.
In a changing and competitive manufacturing economy, there are always new market forces which become the causative factors guiding the design of consumer products. In the case of EXIT, and indeed any electric sign, the cost of powering these appliances has become the prime concern governing their design and marketability. Towards this end, the Light Emitting Diode (LED) exit sign has come into use. These LED signs improve upon the operation of their predecessors. Bulb burnout is essentially eliminated. The light output of LEDs is high, while their power consumption is low, and their low voltage requirement is well suited to battery backup systems. However, there is one aspect to the LED sign which results in a drawback to their usage as a light source for signs of any type, particularly exit signs.
A principal object of this invention is to provide an LED sign which gives the display appearance of the higher power consumption but greater visibility incandescent or florescent signs.
Prior art LED signs utilize many lights (hundreds, in some applications), pointing outwards toward the viewer. Since LEDs are nearly point light sources, like small spot lights, their output being in the shape of a forwardly projecting cone, the appearance of such signs is that of many small dots (which I term "pointillist"). This quality makes it difficult for LED signs to conform to section 5-10.3.4 of the NFPA code described above, when used within their usual design constraints. The "evenly illuminated letters" of this section refers to the type of illumination which was attained in the past when one or several incandescent or florescent light sources were placed in a box with cut out or translucent panels as described above.
In view of the significant advantages realized by the use of LEDs, the NFPA has been allowing their use even though they give a "pointillist" nature to a sign's appearance, and forfeit having the entire face of the sign illuminated by two contrasting colors. The LED signs of the prior art utilize a variety of approaches to come as close as possible to attaining the appearance called for by the NFPA code which, while not stipulating any actual design methods, lead very strongly towards a back lit type of sign using a single or dual light source.
A further object of the invention is to utilize the LEDs in a manner whereby their "pointillist" output will be diffused to give the impression of even illumination of letters.
To approximate, as closely as possible, the appearance of these incandescent signs, manufactures of LED signs have taken two approaches. One is to have the front panel of the sign made of an opaque material, paint it with the desired word and have the center line of each stroke of each letter drilled with holes to accept LEDs penetrating this panel. These signs, because they add no diffusion to the generated light, tend to use LEDs which are themselves diffused to aid in affording as wide a viewing angle as possible. Others place focused LEDs behind a cut-out panel as in the conventional back lit signs previously described, but include another panel of a highly light diffusing material. This second approach actually represents the same approach as in the incandescent signs. There are also signs which place diffused LEDs behind clear panels.
These signs' major drawbacks are that they incorporate significant amounts of light loss due to the fact that they are illuminating the entire interior of the sign's housing. In addition, they are passing the light through multiple layers of materials, incorporating a reflective air space between each. This represents a light loss for each layer due to this reflectance. There are also losses involved due to mismatching between the wavelength of the generated light and the color temperature of the layered materials involved.
Another object of the invention is to provide an LED illuminated sign while minimizing reflective or absorptive losses of the generated light.
The approaches of incorporating LEDs to exit signs described above fall short of fulfilling the NFPA code completely and precisely. They are only capable of illuminating the letter stroke in an insufficient manner, and are incapable of giving illumination in a contrasting color to the area of the sign's face which is not letter stroke (termed the "background").
A further object of the invention is to provide a sign which is capable of illuminating the letter stroke and background area of the sign face separately in two or more contrasting colors.
A number of illuminated signs have been patented. The following examples are considered relevant to the invention.
Turner, U.S. Pat. No. 796,475, represents the most obvious method of illuminating a sign, similar to the large signs commonly seen in amusement parks or advertising signs. Individual lamps are mounted in a reflective channel given the shape of the desired letter. The lamps are incandescent, each having their own socket and wired together in parallel. The resultant appearance of letters constructed in this manner would be for them to have a pointillist quality. In other words, the hot spot of each light bulb would be very evident. FIG. 3 shows that there is some kind of transparent cover over the reflective channel, presumably to either give color to the emitted light, for protection or both. There are a number of disadvantages to this arrangement: Heat will build up, causing a shortened lamp or other component life; When light passes through even an apparently clear substance, there is a light loss of four percent or greater due to reflectance and absorption; and Turner's sign is constructed without the benefit of printed circuit board methods of manufacture, which automate much of the required assembly.
Chao, U.S. Pat. No. 4,028,828, teaches individual letters insertable into a powered track. The letters utilize light bulbs placed in direct line with the viewer's eye, creating hot spots. In this design there is also the condition, even more pronounced than in the above invention, of complete enclosure of the light bulbs, creating heat build up. In claim one of Chaos' invention, it states in line thirteen that the described characters must be hollow, which again creates the condition of the loss of illumination efficiency through reflectance and absorption. While this invention does utilize the advantage of printed circuit board fabrication, the PCB is a separate entity to the housing which comprises the reflector, or the portion of the embodiment which gives shape to the light being emitted.
Schoenfeld, U.S. Pat. No. 4,259,800, approaches the direction in which my invention leans. The characters to be illuminated are given asymmetrically shaped reflective chambers. These chambers are given a basic shape conforming to the major dimensions of height and width of the given character. The condition of the light passing through space and hitting a reflective surface is still evident, creating the transmittance loss mentioned previously. It is also apparent that the bulbs are positioned in a central location, creating hot spots, or more accurately, in this case, dull spots due to the fact that the outer regions of each character are positioned at maximum distances from the bulbs. In the detailed description, column three, third paragraph, Schoenfeld teaches that the lenses should be cast out of a single material with the face of the monolithic structure, and covered with a paint or some other translucent substance. This creates a condition wherein there is a limited amount of bright light being transmitted through this face, giving the brightest illumination to the lenses. The bulbs are in a central location only, creating a different cross section of the reflector at any location within a given cell. There is a sequencing of the characters illuminated. There is mention given to there being the ability of the different lenses changing color, but still the background of each lens will be of the same color, only of a different degree of brilliance.
Plumly, U.S. Pat. No. 4,967,317, shows a sign utilizing incandescent bulbs imbedded in a layer of plastic which diffuses and conducts the light to minimize the pointillist aspect of the bulbs. A face plate has an opaque layer which is cut out to form the letters. Largely, it falls into the same trap of losses due to the reasons mentioned above with the Schoenfeld sign.